Problem: Suppose the diameter of a circle is ${6}$. What is its area?
Solution: ${d = 6}$ ${K = 9\pi}$ First, find the radius: $r = d/2 = {6}/2 = {3}$ Now find the area: $K = \pi r^2$, so $K = \pi \cdot {3}^2 = {9\pi}$.